The present invention relates to a method and apparatus for cancelling an echo, wherein an asymmetrical component in positive and negative transmission pulses can be cancelled so as to realize two-wire bidirectional data transmission.
An echo canceller is conventionally used to achieve two-wire bidirectional data transmission using paired wires. A conventional echo canceller uses an adaptive digital filter for generating an echo replica corresponding to a transmission data sequence having the same length of impulse response as the echo and cancels the echo in accordance with the echo replica. In this case, each tap coefficient of the adaptive digital filter is updated in accordance with correlation between an error signal obtained by subtracting the echo replica from the echo and the transmission data. Assume the echo cancellation in baseband data transmission. AMI (alternate mark inversion) or a biphase coding which has a good DC balance is generally used as transmission line coding. In unipolar/bipolar conversion of AMI coding, a signal of level 0 is generated when a binary value is set at logic "0". Pulses of +V and -V (V&gt;0) levels are alternately generated every time the binary value is set at logic "1". In this case, ideally the +V level positive pulse is symmetrical with the -V level negative pulse. However, the positive and negative pulses are slightly asymmetrical in practice. In the conventional adaptive digital filter based on the assumption that the positive and negative pulses are symmetrical, an asymmetrical component causes an increase in a residual echo level, thus preventing a desired echo suppression. When the impulse response level of the echo at an n-th tap of the filter with respect to the positive pulse is +V', and the level with respect to the negative pulse is -(V'+.beta.) (where .beta..noteq.0), the coefficient of the n-th tap is converged to {V'+(V'+.beta.)}/2=V'+.beta./2. Therefore, the coefficient of the n-th tap is subjected to an error of .beta./2. This ratio can be applied to all tap coefficients. When the positive and negative transmission pulses are asymmetrical, the residual echo level is increased, and desired echo suppression cannot be obtained. For example, in baseband data transmission using paired telephone wires installed between a telephone station and a subscriber, echo suppression of about 50 dB is required. In order to obtain an echo suppression of 50 dB, the symmetry of the positive and negative pulses must be accomplished with a high precision in the order of 99.997%. Although a conventional CMOS gate is used to obtain symmetrical positive and negative voltages, as described in "Large-Scale Integration of Hybrid-Method Digital Subscriber Loops", IEEE Transactions on Communications, Vol. COM-30, No. 9, September 1982, PP. 2095-2108, this CMOS gate is clamped with diodes, so it is difficult to obtain a precise symmetry between the positive and negative pulses. Furthermore, in order to obtain highly precise symmetrical positive and negative pulses, a complicated circuit with many timing points is required. Because of this, circuit adjustment becomes time-consuming, resulting in high cost.